1. Field of the Invention
The present invention relates to a magnetic recording apparatus having a reduced thickness. More specifically, the present invention relates to a thin magnetic recording apparatus which facilitates the assembly and adjustment of an optical unit. Further, the present invention relates to an improved technique for positioning a magnetic head assembly by projecting light beams on a disk-shaped recording medium.
2. Background Art
Most disk-shaped recording mediums prevalently used today are 3.5 inch diameter disks having a high track density in the range of about 2100 to 2500 TPI (tracks per inch) and a storage capacity in the range of 100 to 120 M-byte.
The recording medium is provided with a position sensing track for optical tracking servocontrol for positioning a magnetic head assembly relative to the recording medium to achieve recording information in such a high recording density, or erasing or reproducing information. The position sensing track is used for closed-loop optical servocontrol for positioning the magnetic head assembly relative to recording tracks.
FIG. 7 is a diagrammatic view of an optical unit included in a conventional magnetic recording apparatus. Shown in FIG. 7 are a disk-shaped recording medium 50, a position sensing track 50a of a predetermined length formed continuously in concentric circles on one surface of the recording medium 50, a cartridge 50b for protecting the recording medium 50, and a magnetic head assembly 51 for writing information to the recording medium 50, or erasing or reading information recorded on the recording medium 50. The magnetic head assembly 51 is bonded to a support spring 51a and is mounted on a carriage mechanism, not shown, so that its magnetic gap, not shown, slides on a surface of the recording medium 50 provided with the position sensing track 50a.
A light emitting-and-receiving unit 52 is provided with a laser diode 52a (hereinafter abbreviated to "LD" 52a), i.e., a light source, and a photodiode 52b (hereinafter abbreviated to "PD" 52b), i.e., a photodetection device.
A holographic unit 53 is provided with an optical element, not shown, for dividing light emitted by the LD 52a into a plurality of light beams, and an optical element, not shown, for guiding the reflected light reflected from the recording medium 50 to the PD 52b.
Shown also in FIG. 7 are a lens 54 for condensing light rays from the holographic unit 53 and guiding the reflected light to the holographic unit 53, and a mirror 55 for reflecting a light beam traveled through the lens 54 to the recording medium 50 and guiding the reflected light reflected by the recording medium 50 to the lens 54.
The components 52 to 55 constitute an optical unit. The optical unit, the magnetic head assembly 51 and the magnetic head support spring 51a are carried by a carriage mechanism, not shown, for simultaneous movement in directions parallel to the radius of the recording medium 50.
In operation, the recording medium 50 is rotated at a fixed rotating speed by a recording medium drive motor, not shown. The magnetic gap of the magnetic head assembly 51 supported on the head support spring 51a slides on the recording medium 50.
Closed-loop optical servocontrol will be described. Light emitted by the LD 52a is divided into three light beams by the holographic unit 53, the three light beams are condensed by the lens 54, the three condensed light beams fall on the mirror 55, and then the mirror 55 deflects the three light beams toward the recording medium 50.
The position of the light emitting-and-receiving unit 52 is adjusted so that the three light beams are arranged in a direction at a predetermined angle to the position sensing track 50a when the three light beams are projected on the position sensing track 50a.
The three light beams deflected toward the recording medium 50 by the mirror 55 are reflected toward the mirror 55 in reflected light beams of different intensities dependent on a condition in which the three light beams fall on the recording medium 50 or on the position sensing track 50a of the recording medium 50. The mirror 55 deflects the three reflected light beams toward the lens 54, the three reflected light beams travel through the lens 54 to the holographic unit 53, and then the holographic unit 53 guides the three reflected light beams to the PD 52b.
Upon the reception of the three reflected light beams, the PD 52b generates detection signals respectively representing the intensities of the three reflected light beams. A servocontroller, not shown, receives the detection signals from the PD 52b and gives a drive signal to a carriage drive unit, not shown, for driving the carriage mechanism, not shown, according to the contents of the detection signals. The magnetic head assembly (more precisely, the magnetic gap) carried by the carriage mechanism is positioned relative to a predetermined track by closed-loop optical servocontrol.
In the conventional magnetic head assembly thus constructed, the three light beams must be accurately focused on the recording medium 50 (or the position sensing track 50a). Focusing accuracy must be .+-.50 .mu.m. Focus adjustment in such an accuracy of .+-.50 .mu.m is achieved by adjusting the position of the lens 54. Since the holographic unit 53, the lens 54 and the mirror 55 are separate parts, the positions of those parts relative to each other need fine adjustment taking into consideration the inclination of the lens 54. Thus the focus adjusting work is difficult.
Work for aligning the optical axis of the light beam relative to the position sensing track 50a to enable the light emitting-and-receiving unit 52 to achieve accurate signal detection is very difficult because the light emitting-and-receiving unit 52, the holographic unit 53, the lens 54 and the mirror 55 are separate parts.
Besides position adjustment is difficult, it is also difficult to form the optical unit in a small, thin construction because the holographic unit 53, the lens 54 and the mirror 55 are separate parts and hence the magnetic recording apparatus cannot be formed in a small, thin construction.
Techniques for integrating an optical unit with a holographic unit in a field other than the field of the magnetic recording apparatus are disclosed in JP-A No. 4-219640. However, since those prior art techniques are not those in the field of the magnetic recording apparatus, it is difficult to project a light beam accurately perpendicularly on the position sensing track 50a of the wavering recording medium 50 by those techniques. Therefore, additional parts including mirrors and lenses are necessary for accurately projecting the light beam on the position sensing track 50a and focus adjusting work is difficult.